1. Field of the Invention
The invention is directed to the field of microwave and optical waveguides. More particularly, the invention is directed to methods for designing compact mode control and converter elements for use with microwave and optical waveguides.
2. The Background Art
Mode control and conversion devices find wide application, for example, in microwave heating of plasmas for fusion experiments, multi-mode feeds for RADAR systems, microwave waveguide transitions, couplers and mode filters, microwave waveguide mode launchers for waveguide transmission systems and for laboratory demonstrations, microwave heating systems and optical waveguide couplers, mode converters and mode filters optical and microwave gratings, holographic elements, filters, phase shifters and the like.
Mode converters have been extensively used to convert outputs of high power microwave sources into lower order modes for plasma heating and low loss microwave transmission.
Currently, periodic gratings are used for conversion of modes in highly overmoded circular waveguides. These gratings are formed by periodically varying the waveguide radius resulting in a rippled wall structure and are usually analyzed by coupled mode theory. Such rippled wall structures result in very slight periodic field perturbations. Very high efficiencies have been reported for these gratings, but their lengths remain large compared to the waveguide transverse dimension. Various techniques have been implemented to reduce the length of these gratings, but the overall conversion length remains limited by the grating period. Minimal scattering occurs in such designs and a minimum achievable conversion length appears to be equal to one grating period. See, e.g., K. Kumric et al., "Optimization of Mode Converters for Generating the Fundamental TE.sub.01 Mode from TE.sub.06 Gyrotron Output at 140 GHz," International Journal of Electronics, Vol. 64 (January, 1988), pp. 77-94, and M. J. Buckley et al., "Compact Quasi-Periodic and Aperiodic TE.sub.ON Mode Converters in Overmoded Circular Waveguides for use with Gyrotrons," I.E.E.E. Transactions of Microwave Theory and Techniques, Vol. 38, No. 6 (June, 1990), pp. 712-721. Similar structures in the form of gratings have been designed for optical waveguides. See, e.g., D. Marcuse, "Mode Conversion Caused by Surface Imperfections of a Dielectric Slab Waveguide," Bell Systems Technical Journal (December, 1969) pp. 3187-3215. All of these converter designs are relatively lengthy when compared to the radial dimension of the waveguide. A typical periodic grating mode converter is diagrammed in FIG. 1. In FIG. 1 a first input electromagnetic wavefront 10 is applied to the converter 12. The first wavefront 10 is formed of one or more modes. After interaction with converter 12, which is much longer than it is wide, a second electromagnetic wavefront 14 of the selected modality is output from converter 12. Such converters are typically 95-99.5% efficient.
Mode filters for high power microwave sources whose output power is distributed in various modes, permit extraction of a single mode at the output. Previous designs have not proven themselves particularly efficient. See, e.g., J. P. Tate, et al., "Experimental Proof-of-Principal Results on a Mode-Selective Input Coupler for Gyrotron Applications", I.E.E.E. Transactions on Microwave Theory and Techniques", Vol. 42, No. 10 (October, 1994), pp. 1910-1917, and U.S. Pat. No. 3,771,078 dated Nov. 6, 1973 to H. G. Kidner, et al.
Mode launchers for efficiently exciting a specific mode into an overmoded waveguide can be difficult to construct. However, for a single mode waveguide only one mode survives and thus the mode purity is ensured.
Waveguide adapters, such as tapered sections, are generally used to join waveguides of unequal radial dimensions. Due to a gradual taper these devices are very long as compared to the radial waveguide dimension. See, e.g., W. A. Huting et al., "Numerical Solution of the Continuous Waveguide Transition Problem," I.E.E.E. Transactions on Microwave Theory and Techniques, Vol. 36, No. 11 (November, 1989), pp. 1802-1818.
Grating couplers can be used to couple free space light into an optical waveguide or vice versa and also for coupling between adjacent waveguides. See, e.g., Nishihara, Haruna and Suhara, "Optical Integrated Circuits", McGraw-Hill Optical and Electro-optical Engineering Series, 1989.
Accordingly, a method for designing more compact, yet equally efficient mode converters and control elements for microwave and optical applications would be highly desirable.